1. Field of the Invention
The present invention is directed to a photomask used in photolithography process in fabricating an LSI device and a method of manufacturing the same.
2. Description of the Background Art
A photomask disclosed in Japanese Patent Application Laid-Open Gazette No. 62-66444 can be cited as an example of a conventional photomask. As shown in FIG. 1, the conventional photomask comprises a glass substrate 50, an anti-reflection film 51 and a light shielding pattern 52, the front surface of the glass substrate 50 being covered with an anti-reflection film 51, the light shielding pattern 52 being formed on the surface of the anti-reflection film 51. The anti-reflection film 51 has a minimum reflectance ratio when its refractive index n.sub.a and its thickness d.sub.a are: EQU n.sub.a .multidot.d.sub.a =.lambda./4 (1)
The minimum reflectance ratio of the anti-reflection film 51 becomes zero when: EQU n.sub.a.sup.2 =n.sub.air .multidot.n.sub.g ( 2)
where n.sub.g is the refractive index of the glass material of the glass substrate 50 and n.sub.air is the refractive index of air.
FIG. 3 is a diagram of an optical system for printing a circuit pattern on a wafer W by using such a conventional photomask P'. In FIG. 3, a light source is labelled G and lenses are labelled L.sub.1 and L.sub.2.
The conventional photomask P' has a problem as explained in the description below. For convenience of understandings, the description below is on the premise that the film thickness of the anti-reflection film 51 fully satisfies Eq. 1.
First, explanation is given on where the refractive index of the anti-reflection film 51 is smaller than that of the glass substrate 50, and not to mention, smaller than that of the light shielding pattern 52. In this case, a light beam incident upon the photomask P' advances as shown in FIG. 2A. More precisely, an incident light beam A' entering the glass substrate 50 from the back surface side is partially reflected at an interface 53 between the glass substrate 50 and the anti-reflection film 51 and becomes a reflected light beam B'. Since the refractive index of the anti-reflection film 51 is smaller than that of the glass substrate 50, the reflection at the interface 53 does not cause phase shift. An incident light beam A' entering the anti-reflection film 51 is reflected at an interface 54 between the anti-reflection film 51 and air and becomes a reflected light beam C' or reflected at an interface 55 between the anti-reflection film 51 and the light shielding pattern 52 and becomes a reflected light beam D'. Again, phase shift does not occur at the interface 54 because the refractive index of the anti-reflection film 51 is larger. than that of air (See the imaginary lines of FIGS. 2A and 2B). On the other hand, the refractive index of the anti-reflection film 51 is generally smaller than that of the light shielding pattern 52. Hence, where .lambda. is a wavelength as a light beam has in air, a light beam reflected at the interface 55 is .lambda./2 out of phase from the incident light beam A'. As mentioned earlier, the thickness of the anti-reflection film 51 is so determined that Eq. 1 is satisfied. Hence, when a light beam enters the anti-reflection film 51 and is reflected at the interface 54 or the interface 55, the phase of the reflected light beam is led by .lambda./2 in terms of an optical path length within the anti-reflection film 51. As a result, once the reflected light beams B' and C', which are not phase shifted when reflected, have entered the glass substrate 50, they have reversed phases shifted by .lambda./2. Due to the reverse phases, the reflected light beams B' and C' cancel each other, whereby reflected light beams advancing within the glass substrate 50 are restrained. 0n the contrary, since the reflected light beam D' is phase shifted by .lambda./2 when reflected, the reflected light beams B' and D' have same phases in the glass substrate 50, hence intensifying each other. As a result, intensified strong reflected light beams pass through the glass substrate 50. More particularly, when a light beam is reflected at the light shielding pattern 52, the reflected light beam advances within the glass substrate 50 as intensified light beam. Such a reflected light beam with a strengthened intensity is problematic. Chances are that the reflected light beam is reflected once again at the interface 56 between the back surface of the glass substrate 50 and air, and goes out of the glass substrate 50 from a region between adjacent light shielding patterns 52. The reflected light beam allowed to the outside of the glass substrate 50 in such a manner is a stray light beam, which deteriorates a printed pattern.
Now, description is directed to where the anti-reflection film 51 is made of material which has a refractive index smaller than that of the light shielding pattern 52 but larger than that of the glass substrate 50. In this case, propagation of light beams is as shown in FIG. 2B. Unlike the case shown in FIG. 2A, an incident light beam A' is partially reflected at the interface 53 between the glass substrate 50 and the anti-reflection film 51 while phase shifted by .lambda./2 and becomes a reflected light beam E'. At the interface 54 between the anti-reflection film 51 and air, the incident light beam A' is reflected without being phase shifted and becomes a reflected light beam F'. At the interface 55 between the anti-reflection film 51 and the light shielding pattern 52, the incident light beam A' is reflected while being phase shifted by .lambda./2 and becomes a reflected light beam G'. Since the thickness of the anti-reflection film 51 is so determined that Eq. 1 is satisfied, the reflected light beams E' and G', which are phase shifted by .lambda./2 from the incident light beam A' when reflected, have reversed phases shifted by .lambda./2 from each other so that they cancel each other within the glass substrate 50'. However, since the reflected light beam F' is not phase shifted when reflected, the reflected light beams E' and F' have same phases and intensify each other within the glass substrate 50. Thus, stray light beams result similarly to the case of FIG. 2A, and cause deterioration in a printed pattern on a wafer.
The present invention has been made to solve such problems. The object of the present invention is therefore to offer a photomask which restrains reflected light beams with an increased effect thereby to prevent deterioration in a printed pattern on a wafer. To offer a method of manufacturing such photomask is also what the present invention intends to achieve.